Four-level voltage supply for liquid crystal display

ABSTRACT

A power supply circuit supplies a liquid crystal energizing circuit with desired potentials for the purposes of energizing a liquid crystal display in accordance with combinations between first, second, third and reference potentials. The power supply circuit includes a first input terminal connected to a constant voltage source for supplying the first potential, a second input terminal connected to the reference potential, first, second, third, fourth and fifth output terminals for supplying the liquid crystal energizing circuit with desired potentials, impedance means connected between the first input terminal and the second input terminal for deriving the second potential and the third potential therefrom, means for always supplying the first output terminal and the fifth terminal with the first potential and the reference potential respectively, and switching means for determining whether the second, third and fourth output terminals are respectively with the first, second and third potentials or with the second, third and reference potentials. The last named switching means contain only P channel MOS transistors (or N channel MOS transistors).

BACKGROUND OF THE INVENTION

The present invention relates to a power supply circuit for providing a liquid crystal display with desired voltage levels. More particularly, the present invention relates to the power supply circuit of the above described type which can simplify circuit configuration and then facilitate fabrication of the power supply circuit.

The inventors have proposed an earlier power supply circuit suitable for supplying a liquid crystal energizing circuit with desired voltage levels or potentials as shown and described in copending application, FOUR-LEVEL VOLTAGE SUPPLY FOR LIQUID CRYSTAL DISPLAY, Ser. No. 685,261, now U.S. Pat. No. 4,050,064, filed May 11, 1976 by Shintaro Hashimoto and Yuuichi Sato and assigned to the same assignee as the present invention, the disclosure of which is incorporated herein by reference. The earlier circuit as shown in FIG. 1, includes an input terminal In₁ connected to a reference potential or OV, another input terminal In₂ connected to a constant voltage source VC and output terminals a-e for providing the liquid crystal display energizing circuit with desired potentials. The input terminal In₁ is connected directly to the output terminal a and the second input terminal In₂ is connected directly to the output terminal e. Resistors R₁, R₂, R₃ and R₄ of the substantially same resistance value are serially connected between the input terminals In₁ and In₂ and the output terminals b, c and d are coupled with the respective middle points of the series circuit of R₁, R₂, R₃ and R₄.

There is also provided a complementary MOS circuit which comprises a P channel MOS transistor Tr₈ connected in parallel with the resistor R₁ and an N channel MOS transistor Tr₉ connected in parallel with the resistor R₄. The transistors Tr₈ and Tr₉ are switchable between ON and OFF states in response to control signals A.

When the control signal A is OV, the transistor Tr₉ is ON to establish a short circuit between In₂ and d such that voltage between In₁ and In₂ is divided through the use of the resistors R₁, R₂ and R₃ to produce outputs VA, VB and VC via terminals b-d. It is concluded that a = OV, b = VA, c = VB and d = e = VC. For the DSM type of liquid crystal displays with an ignition voltage of 18V, VC = -18V, VB = -12V and VA = -6V.

Conversely, when the control signal A is VC (-18V), the transistor Tr₈ is ON to establish a short circuit between In₁ and b such that voltage between In₁ and In₂ is divided by R₂, R₃ and R₄ thereby to produce outputs VA and VB via C and D at terminals c, d, and e. As a consequence, a = b = OV, c = VA, d = VB and e = VC.

Nevertheless, although the above described power supply circuit including as the switching means the complementary MOS circuit as shown in FIG. 1 is advantageous from the viewpoint of circuit technique since the complementary MOS circuit provides output voltage levels approximately equal to the input voltage levels, it is still difficult to fabricate the complementary MOS circuit configuration at a low cost.

Accordingly, it is an object of the present invention to provide an improvement in the earlier proposed liquid crystal display driving circuit which does not include a complementary MOS transistor circuit configuration as a switching means. In accordance with the concept of the present invention, the power supply circuit for the liquid crystal display energizing circuit is adapted in such a way as to constitute the switching means by only P channel MOS transistors (or N channel MOS transistors).

BRIEF DESCRIPTION OF THE DRAWING

A better understanding of the present invention may be had from a consideration of the following detailed description taken in conjunction with the accompanying drawing in which:

FIG. 1 is a circuit diagram of a power supply circuit using complementary MOS transistors;

FIG. 2 is a circuit diagram of a power supply circuit constructed in accordance with one preferred form of the present invention;

FIG. 3 is an explanatory diagram for the purposes of illustrating the operation of the circuit of FIG. 2;

FIG. 4 is a circuit diagram of another preferred form of the present invention;

FIG. 5 is a circuit diagram of still another preferred form of the present invention;

FIG. 6 is a circuit diagram of the portion A of the circuit of FIG. 5;

FIG. 7 is a timing diagram of waveforms of signals which occur within the circuit of FIG. 6;

FIG. 8 is a block diagram of a peripheral configuration of the power supply circuit of the present invention;

FIG. 9 is a circuit diagram of a circuit providing segment signals for a liquid crystal display; and

FIG. 10 is a timing diagram of signals which occur within the circuit of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 2, there is illustrated a power supply circuit constructed in accordance with the present invention which includes the same parts as shown in FIG. 1, that is, the input terminals In₁, In₂, the output terminals a-e and the resistors R₁, R₂, R₃, R₄. While the P channel MOS transistor Tr₈ is connected in parallel with the resistor R₁ in the similar manner, a P channel MOS transistor Tr₉ ' (not N channel MOS transistor as in FIG. 1) is connected in parallel with the resistor R₄. The control signal A is applied to the gate of the transistor Tr₈ and the inverted signal via an inverter is applied to the gate of the transistor Tr₉ '. It will be noted that the above inverter comprises P channel MOS transistors Tr₁₀, Tr₁₁. An additional resistor R₅ is provided in series with the resistor R₄ and the P channel transistor Tr₉ ' .

In the case where as in FIG. 2 the switching means are constituted by the P channel MOS transistors Tr₈ and Tr₉ ', when the control signal A is OV, the transistor Tr₉ ' is ON to short between In₂ and d. When the control signal A is VC', the transistor Tr₈ is ON to short between In₁ and b.

As viewed from FIG. 3, when the gate is -V and the source is -V for the P channel MOS transistors, V_(OUT) will be reduced by the threshold voltage V_(t) and thus assume - (V - V_(t)). This implies that -V cannot be outputted as V_(OUT). It will be understood that with the complementary MOS transistor configuration set forth in the foregoing paragraphs with respect to FIG. 1, -V can be completely outputted as V_(OUT).

To this end, in FIG. 2 the source of the transistor Tr₉ ' is connected to the point f (the junction with the output terminal e). If the voltage fV at that point is for example -6V, the gate voltage of the transistor Tr₉ ' should be negative with respect to the source voltage namely -6V by at least the threshold voltage (generally, -2 to -3V) in order to derive -6V via the drain of the transistor Tr₉ ' or d. The OFF output voltage of the inverter of Tr₁₀ and Tr₁₁ should be therefore decreased to such extent. The OFF output voltage of the inverter is determinative upon the constant voltage source VC' at In₂ and therefore is VC'-V_(t). VC' is preliminarily established to be at least fV + 2V_(t) and thus -6V of fV is outputted via d.

The resistor R₅ provided between In₂ and f is selected to meet the predetermined interrelationship between VC' of In₂ and fV of f. In other words, when Tr₈ is ON and Tr₉ is OFF the potential fV at the point f is written as follows: ##STR1## And when Tr₈ is OFF and Tr₈ ' is ON the potential fV is as follows: ##STR2##

It will be obvious from the above equations that if R₁ = R₄ is satisfied, fV becomes fixed. Proper choice of the resistor R₅ enables establishment of desired interrelationship between VC' and fV. A Zener diode may be employed instead of the resistor R₅ to set fV of the point f.

In FIG. 2, VC' of In₂ is, in fact, selected at -16V and the output of Tr₁₀ is about -12 to -13V. Accordingly, since sufficiently low voltage is applied to the gate of Tr₁₀, -6V of f can be derived from d without any variations therein.

With such an arrangement, when the control signal A is OV, Tr₉ ' is ON to establish a short circuit between In₂ and d such that voltage division through the resistors R₁, R₂ and R₃ results in d = e = -6V, c = -4V, b = -2V and a = OV. The liquid crystal display of the FEM type is ignited upon application of 6V. Conversely, when the control signal A is VC', Tr₈ is ON to short the circuit between In₁ and b such that a = b = OV, c = -2V, d = -4V and e = -6V through the use of the voltage divider of R₂, R₃ and R₄.

FIG. 4 is a modification in the circuit of FIG. 2 wherein the inverter circuit is implemented with a MOS device of the ED (enhancement/depletion) type. Tr₁₀ is of the enhancement type while Tr₁₁ is of the depletion type. Since the gate voltage of Tr₉ ' is in proximity to VC' of In₂, it is not necessary that VC' be negative to such extent as discussed above.

FIG. 5 shows still another preferred form of the invention which resembles that of FIG. 2 with exception of the transistor Tr₉ ' and the inverter circuit. The portion including the transistor Tr₉ ' and the inverter circuit is denoted as circuit A'.

The circuit A' is of the circuit configuration as shown in FIG. 6 wherein P channel MOS transistors Tr₉ ' and Tr₉ " are connected in parallel with the sources connected to the point f (the junction with the output terminal e) and the drains connected to the junction with the output terminal d.

The circuit A' comprises a pair of bootstrap circuits X and Y coupled to output fV of the point f via the output terminal d and 2/3fV via the output terminal d.

On the first bootstrap circuit X there is provided an inverter circuit of P channel MOS transistors Tr₁₂ and Tr₁₃ at a one terminal of a capacitor C₁, the one terminal of the capacitor C₁ being biased through a MOS resistor of Tr₁₂ and the other terminal thereof being switchable through a MOS circuit of the ED type consisting of transistors Tr₁₄ and Tr₁₅. Repetition signals a₁ of relatively high frequency are supplied for Tr₁₅.

To this end, the output voltage of Tr₁₅ and in other words the other terminal of the capacitor C₁ assumes OV and VC' (for example -6V). When OV, the capacitor C₁ is charged to about |VC'|. At a moment that the output of Tr₁₅ is changed from OV to VC', the potential of A₁ (the one terminal of the capacitor C₁) will be changed from VC' to 2VC'. If the output of A₁ is applied to the gate of Tr₉ ' to effectively utilize such variation to 2VC', VC' of the point f can be outputted via the output terminal d because of the gate held at 2VC'.

The potential of 2VC' is not of a permanent character and therefore varies in accordance with developments of the discharge procedure. Therefore, Tr₉ ' is rendered ON for only a short period of time. For this reason VC' or 2/3VC' cannot be continuously outputted via the output terminal d. The P channel MOS transistor Tr₉ " of the second bootstrap circuit Y, therefore, is connected in parallel with Tr₉ '. The output of A₂ applied to the gate of Tr₉ " permits VC' of the point f to be outputted via the output terminal d without variations and therefore VC' or 2/3VC' to be permanently outputted via the output terminal d in cooperation with the first bootstrap circuit X.

The output of A₂ set forth above is obtainable from the second bootstrap circuit Y in the same manner as that of the circuit X. The repetition signals a₂ are applied to Tr₁₉ within the MOS circuit device of the ED type.

FIG. 7 is a timing diagram showing waveforms of signals within the circuit of FIG. 6. The signals a₁ and a₂ are of the waveforms designated a and b such that periods of time for remaining OFF state of Tr₁₅ and Tr₁₉ are different but somewhat overlapped with each other. c designates the waveform of the control signal A, d designates that of the output of A₁, e designates that of the output of A₂ and f designates that of the output of the output terminal d.

When the control signal A is OV, Tr₁₂ and Tr₁₆ are ON. When the a₁ signal is OV, the output of Tr₁₅ is OFF and hence VC'. The potential of A₁ falls from VC' to 2VC' to make Tr₉ ' ON. If the a₂ signal is OV, Tr₁₉ is OFF and its output is VC'. As a result, the potential of A₂ falls from VC' to 2VC' thereby to render Tr₉ " ON. As ON period for Tr₉ ' is placed to overlap with that for Tr₉ ", fV = VC' of the point f is successively developed at the output terminal d as suggested by f.

On the other hand, when the control signal A is VC', Tr₁₃ and Tr₁₇ are ON to keep the potentials of A₁ and A₂ at OV and in addition to render Tr₉ ' and Tr₉ " OFF. This is the correspondence to that the circuit A' of FIG. 5 is rendered OFF.

In summary, when the control signal A is OV, Tr₉ ' and Tr₉ " are ON to establish shorted circuit between In₂ and the output terminal d, followed by that voltage division is effected by the resistors R₁, R₂ and R₃ and consequently d = e = -6V, c = -4V, b = -2V and a = OV. Conversely, when the control signal A is VC', Tr₈ is ON and circuit between In₁ and b is shunted such that voltage division by the resistors R₁, R₂ and R₃ results in a = b = OV, c = -2V, d = 4V and e = -6V. Provision of the constant voltage source of more negative voltage as shown in FIG. 2 is not needed due to a parallel combination of the two bootstrap circuits X and Y as shown in FIG. 6. Therefore, a constant voltage source of -6V can be employed (in FIG. 2 -16V) and Tr₈ and Tr₉ ' as switching means can be implemented with a P channel MOS transistor.

As depicted by f showing the waveform of the output at the output terminal d in FIG. 7, under the conditions the control signal A is OV, the signal a₁ is VC' and the signal a₂ is OV at the time t₀, Tr₈, Tr₉ ' and Tr₉ " are to be OFF (as A₂ is not 2VC'). Thus, fV of the point f has to be outputted via the output terminal d since Tr₈ and the circuit A' are both placed in OFF state in FIG. 5. However, there is created the possibility of providing undesired potentials. By sufficiently increasing frequency of the signals a₁ and a₂ as compared with frequency of the control signal A, the possibility can be avoided or ignored.

Although in the circuit of FIG. 6 Tr₁₄ and Tr₁₅ are of the ED type MOS configuration, they may be in the same form as the P channel MOS transistor Tr₁₂.

FIG. 8 is a schematic of a peripheral circuit arrangement which utilizes the potential distributing circuits stated above with respect to FIGS. 2, 4 and 5. By way of a one chip MOS/LSI calculator, an LSI chip internally contains the potential distributing circuit in addition to a conventional arithmetic control circuit. The LSI chip is supplied with VC' from the constant voltage source. And, as obvious in the art of calculators, the LSI chip receives key signals from a keyboard and provides display signals for a liquid crystal display. The constant voltage source VC' is coupled as V_(DD) to enable terminals of the LSI chip to energize P channel MOS transistors within the control circuit.

For example, in the case where VC' of -16V as shown in FIG. 2 is employed, P channel MOS transistors within the control circuit are of the high threshold type powered with -16V, thereby reducing the number of power or enable terminals of the LSI chip as small as possible. In many cases, the LSI chip needs V_(GG) power source somewhat smaller than V_(DD) for establishment of clock pulse levels but VC' may be the correspondence to V_(GG).

FIG. 9 shows an example of a circuit arrangement adapted for providing segment signals SI in response to the potentials from the potential distributing circuit. This comprises a couple of bootstrap circuits M and N and resembles essentially the circuit arrangement shown in FIG. 6 wherein the P channel MOS transistors Tr₁ and Tr₂ are connected in parallel. The commonly connected sources of Tr₁ and Tr₂ are led to the output terminal of the segment signal SI and the drain of a P channel MOS transistor Tr₃. The drains of Tr₁ and Tr₂ are connected to the output terminal d of the potential distributing circuit. Tr₃ has its source connected to the output terminal b of the potential distributing circuit and its gate receiving segment selection signals s. The circuit arrangement including a capacitor C₁, Tr₁₂ to Tr₁₅ within the bootstrap circuit M and a capacitor C₂, Tr₁₆ to Tr₁₈ within the bootstrap circuit N, operates in the same mode as that of FIG. 6.

A time diagram of the waveforms of signals in operation of the circuit of FIG. 9 is shown in FIG. 10. a designates the signal a₁ applied to the gate of Tr₁₅, b designates the signal a₂ applied to the gate of Tr₁₉, c designates coincidence between the control signal A and the segment selection signal s which is derived via a decoder from a register, d designates the waveform of the output of A₁, e designates the waveform of the output of A₂ and f designates the waveform of the segment signal SI. The segment SI assumes either one of the potentials supplied from the output terminals b and d in accordance with the potential of the segment selection signal s.

Although there has been described above a specific arrangement of the liquid crystal power supply circuit in accordance with the invention for the purpose of illustrating the manner in which the invention may be used to advantage, it will be appreciated that invention is not limited thereto. Accordingly, any modifications, variations or equivalent arrangements which may occur to those skilled in the art should be considered to be within the scope of the invention. 

What is claimed is:
 1. A power supply circuit for supplying a liquid crystal display energizing circuit with desired potentials for the purposes of energizing a liquid crystal display in accordance with combinations between first, second, third and reference potentials, comprising:a first input terminal for supplying the first potential, connected to a constant voltage source; a second input terminal connected to the reference potential; first, second, third, fourth and fifth output terminals connected for supplying the liquid crystal display energizing circuit with desired potentials; impedance means connected between the first input terminal and the second input terminal for deriving the second potential and the third potential therefrom; means for always supplying the first output terminal and the fifth output terminal with the first potential and the reference potential, respectively; and switching means for controlling whether the second, third and fourth output terminals are respectively supplied with the first, second and third potentials or with the second, third and reference potentials; said switching means comprising field effect mode transistors of the same channel conductivity type.
 2. A power supply circuit as set forth in claim 1 wherein the field effect mode transistors are of the P channel MOS type.
 3. A power supply circuit as set forth in claim 1 wherein the field effect mode transistors are of the N channel MOS type.
 4. A power supply circuit as set forth in claim 2 wherein said switching means comprises a pair of P channel MOS transistors.
 5. A power supply circuit as set forth in claim 4 wherein one of the two P channel MOS transistors has a source-to-drain circuit connected between the first input terminal and the second output terminal and the other of the two P channel MOS transistors has a source-to-drain circuit connected between the second input terminal and the fourth output terminal.
 6. A power supply circuit as set forth in claim 5 wherein impedance means are provided between the first input terminal and the source-to-drain circuit of said one of the two P channel MOS transistors.
 7. A power supply circuit as set forth in claim 6 wherein an inverter circuit is provided and in circuit with the said one of the two P channel MOS transistors.
 8. A power circuit as set forth in claim 7 wherein the inverter comprises a couple of P channel MOS transistors.
 9. A power supply circuit as set forth in claim 8 wherein the P channel MOS transistors are of the enhancement/depletion type.
 10. A power supply circuit as set forth in claim 1 wherein said switching means comprises a pair of bootstrap circuits.
 11. A power supply circuit as set forth in claim 10 wherein each of said bootstrap circuits has a capacitor adapted to be biased.
 12. A power supply circuit as set forth in claim 11 wherein each of said bootstrap circuits has a field effect mode transistor to control charging and discharging of said capacitor. 